This invention relates to FM stereophonic broadcasting systems and, more particularly, to an improved FM stereophonic broadcasting system utilizing companding of the difference signal and simultaneous companding and modulation/demodulation techniques.
U.S. Pat. No. 4,485,483 of Emil L. Torick and Thomas B. Keller, the disclosure of which is hereby incorporated herein by reference, describes an FM stereophonic broadcasting system in which the usual left and right signals are conventionally matrixed to obtain conventional sum (M) and difference (S) signals, and the difference signal is used to amplitude-modulate a first sub-carrier signal and is also applied to a compressor which compresses its dynamic range in accordance with a given law to produce a compressed difference signal S'. The compressed difference signal S' amplitude-modulates a second sub-carrier signal, preferably of the same frequency but in quadrature phase relationship with the first. The M signal, the two modulated sub-carrier signals, and a pilot are then frequency modulated onto a high frequency carrier for transmission purposes. The receiver includes a demodulator for deriving the M signal, the normal difference signal S and the compressed difference signal S', and an expander for complementarily expanding the derived compressed difference signal. The expanded noise-reduced version of the difference signal is combined with the derived sum signal M to obtain the original left (L) and right (R) signals. Companding of the difference signal S gives 22 db to 26 db signal-to-noise improvement in the transmission thereby to greatly extend the effective stereo service area over that of the existing FM stereo service.
Commonly assigned U.S. patent application Ser. No. 688,940 filed Jan. 4, 1985, by applicant David W. Stebbings, now U.S. Pat. No. 4,602,380 the disclosure of which is hereby incorporated herein by reference, discloses an improvement of the above-described Torick/Keller system in that unlike the Torick/Keller system, in which only the expanded version of the received compressed difference signal is matrixed with the sum signal M to obtain the original L and R signals, the receiver matrixes a noise-reduced difference signal, derived by combining and expanding the received difference signals S and S', with the derived signal M to obtain the original L and R signals. Since the signal content of the signals S and S' is the same, by combining them the effective level of the received difference signal is increased by about 6 db, whereas the noise is increased by only about 3 db, resulting in a net improvement in signal-to-noise ratio of about 3 db over that of the Torick/Keller system. The transmission of the uncompressed difference signal S (necessary for compatibility with existing systems), coupled with the utilization at the receiver of both the uncompressed and compressed difference signals, allows the use of any desired companding law for the compression of the difference signal.
Commonly assigned U.S. patent application Ser. No. 688,854, also filed Jan. 4, 1985, by Aldo G. Cugnini, Daniel W. Gravereaux and applicant David W. Stebbings, now U.S. Pat. No. 4,602,381 the disclosure of which is also hereby incorporated herein by reference, describes an FM stereophonic broadcasting system which utilizes the Torick/Keller concept of transmitting both the usual difference signal S and a compressed difference signal S' and the Stebbings concept of combining the usual difference signal and the expanded version of the compressed difference signal at the receiver to achieve greater noise reduction and improved signal-to-noise ratio, and additionally uses the normal difference signal at the receiver as a reference signal for controlling the expander so as to cause the amplitude of the companded difference signal to equal the level of the normal difference signal and thus insure proper dematrixing with the received sum signal M. The latter feature has the important advantage that the expander is adaptive to any compressor characteristic that might be employed at the transmitter. Further, the uncompressed difference signal available at the receiver enables adaptive decoding of dynamic parameters of the received signal, such as frequency response and attack and recovery time constants, so that all of the parameters of the original signal can be substantially restored, automatically, whatever the transmission system used.
In order that the adaptive expander system described in the aforementioned application Ser. No. 688,854 realize the maximum benefit from the availability at the receiver of both the unchanged and compressed difference signals S and S' it was necessary to use a stereo decoder (or demultiplexer), often in integrated circuit form, modified to give access to both the S and S' signals in quadrature. More particularly, in the prior system, these signals and decoded from the 38 KHz carrier to derive the corresponding baseband audio difference signals, both unchanged (S) and compressed (S'). The expander, which in a preferred embodiment is adaptive in nature, operates on the demodulated S and S' signals, with the S signal utilized as a reference for controlling the expansion. Thus, the decoder IC requires an extra demodulator as compared to the conventional FM stereo receiver, and also a second reference 38 KHz carrier, either in quadrature or at 45.degree. for driving the extra demodulator, as well as other matrices and components for effecting demodulation and expansion of the compressed difference signal. These additional components represent an increase in complexity and cost over that of conventional FM stereo receivers, and in order for the improved system to be more readily accepted these added costs are desirably reduced to more nearly correspond to the cost of conventional receivers.
Accordingly, it is a primary object of the present invention to provide an FM stereophonic broadcasting system utilizing companding of the difference signal which is simpler and less costly than the systems described in the aforementioned copending patent applications.
Another object of the invention is to provide an improved companding system having lower distortion level, and less control signal breakthrough to the audio output.
Yet another object of the invention is to provide in an FM stereophonic receiver for a system utilizing companding of the difference signal in which simultaneous demodulation and expansion of the difference signal is combined with the simultaneous demodulation of the FM signal directly at the output of the FM limiter of the receiver.
A more specific object of the invention is to provide a technique for simultaneously modulating an audio signal onto a reference carrier and compressing the signal, or for simultaneously demodulating and expanding such compressed signal.